Energy efficiency has recently taken on a great deal of importance in Information Technology. Two major intertwined issues have driven this switch: (1) the realization that emission of Green-House Gases (GHGs), such as carbon dioxide, into the atmosphere threatens our well-being. As a major, long-term threat to humanity, much has been written on this subject. The management of GHGs lies at a volatile intersection of politics, economics, international relations and ecology. It is clear that policy makers worldwide will struggle with this problem for years to come.